As for a process for chemically etching and removing an oxide film formed on a wafer as a substrate, there is known, e.g., COR (Chemical Oxide Removal) processing (see, e.g., U.S. Patent Application Publication No. 2004/0185670). In the COR processing, the oxide film formed on a surface of the wafer is made to react with hydrogen fluoride gas and ammonia gas, and ammonium hexafluorosilicate (AFS) is generated as a reaction product from the oxide film. The generated AFS is sublimated and removed by heating the wafer. In the COR processing, the amount of AFS is increased as time elapses. Therefore, the removal amount (etching amount) of the oxide film is controlled by controlling a processing time of the COR processing.
The COR processing may be repeatedly performed on the wafer. However, it has been known that the etching amount of the oxide film in each COR processing is not uniform depending on a so-called Q-time which is a period of time from previous COR processing to current COR processing. In order to reliably etch the oxide film by a desired amount, the processing time of the COR processing is set to be longer than the processing time required for etching the oxide film by the desired amount.
However, if the processing time of the COR processing is set to be longer than the processing time required for etching the oxide film by the desired amount, the etching amount of the oxide film may exceed the desired amount and the oxide film may be etched more than required. For example, when forming a hole in the oxide film, even though the processing time is set such that the hole does not penetrate through the oxide film, the etching amount of the oxide film may exceed the desired amount and the hole may penetrate through the oxide film. Therefore, it is difficult to accurately control the etching amount of the oxide film.